1. Field of the Invention
The present invention relates to a photographic film. More particularly, the present invention relates to a photographic film which is wound about a spool and contained in a photo film cassette.
2. Description Related to the Prior Art
Photographic film of the 135 type has two trains of perforations arranged along respective lateral edges and at a regular pitch, and contained in a cassette shell of a photo film cassette having light shielding capacity. A trailer of the photo film is connected to a spool in the cassette shell, and is contained in the cassette shell after winding the photo film in a roll form.
In FIG. 8, a trailer of a photo filmstrip 102 is illustrated. Perforations 4 are arranged at a regular pitch P. A trailer of the photo filmstrip 102 has a retainer portion 103, which is narrower than the remaining portion of the photo filmstrip 102. The retainer portion 103 has a length A and a width W. To define the length A of the retainer portion 103, lateral edges are cut along a first line 120 being perpendicular to the lateral edges. The first line 120 has a position lying between the perforations 4. To define the width W of the retainer portion 103, the trailer is cut along second and third lines 121a and 121b being parallel to the lateral edges.
Two retaining holes 3a are formed in the retainer portion 103. When the retainer portion 103 is inserted in a slot 6 in a spool core 5, retaining claws 7 inside the slot 6 are engaged with the retaining holes 3a. Then the trailer is connected to the spool core 5, which is rotated in orienting the emulsion surface toward the spool core 5. The photo filmstrip 102 is wound on the spool core 5 in a roll form between flanges 5a.
In FIG. 9, continuous photo film 8 for producing the photo filmstrip 102 is illustrated. When the continuous photo film 8 is conveyed in the direction of the arrow at an amount as much as predetermined, the continuous photo film 8 is stopped and a cutter or trimmer is operated for forming a leader of a preceding photo filmstrip 102a and the trailer of the photo filmstrip 102 of FIG. 8, while eliminating a waste of the photo film. A pair of retaining holes 3a are formed by the cutter at the same time.
In the photo filmstrip 102, the first line 120 has a position between the perforations. In FIG. 8, the photo filmstrip 102 meets t2&lt;t, where t is an interval between the perforations 4, and t2 is an interval between the first line 120 and one of the perforations 4 the closest to the end of the trailer. A shape of the cutter for shaping both the trailer of the photo filmstrip 102 and the leader of the photo filmstrip 102a is determined by considering the position of the first line 120.
The photo filmstrip 102 about the spool core 5 is contained in a cassette shell light-tightly (See FIGS. 3 and 4). Initially the leader of the photo filmstrip 102 is protruded from a photo film passage port. To load a camera with the photo filmstrip 102, the leader is manually picked and pulled as much as required. An end of the leader is fitted on a take-up spool of the camera. If a camera having an auto-loading mechanism is used, the leader end is placed near to the take-up spool suitably.
A user may rotate the spool core 5 opposite to a direction of photo film winding, with incidental intention to draw out the leader as much as required. If the leader is drawn out longer than required, the spool core 5 can be rotated in the winding direction for the purpose of winding back a surplus portion of the leader. If the user is unaccustomed to the use of the photo film cassette, he or she is likely to rotate the spool core 5 opposite to the winding direction. As the leader of the photo filmstrip 102 is in frictional contact with light-trapping fabric in the passage port, the roll of the photo filmstrip 102 becomes loosened inside the cassette about the spool core 5 being rotated. A gap L is formed between the spool core 5 and an innermost one of the turns of the photo filmstrip 102 (See FIG. 6).
The spool core 5 is rotated further in the direction opposite to the direction of winding of the photo filmstrip 102. A portion of the photo filmstrip 102 protruded from the slot 6 is pressed by force in a direction of orienting the emulsion surface convexly. A point H lying on the first line 120 is indicated in FIG. 10A. The portion at the point H is pushed by an open edge of the slot 6, so that the photo filmstrip 102 receives strong force in the opposite rotational direction inside the gap L (See FIG. 6).
The photo filmstrip 102 kept in the roll form has the irresistible tendency of maintaining a curl in a longitudinal direction. The photo filmstrip 102 also has a curling tendency in a width direction, like an archway. Upon application of the force in the opposite rotational direction against the curling tendency of the innermost turn of the photo filmstrip 102, the portions at the first line 120 flex or bend as illustrated in FIG. 10B. Stress occurs in directions of the arrows in the drawing at the perforations 4. The photo filmstrip 102 is likely to break from a corner of one of the perforations 4 along the broken line indicated in the drawing, only upon small reverse rotation of the spool core 5 with occurrence of the gap L (See FIG. 6).
The gap L is unfavorable especially when small. As an outer shape of the cassette shell is unchanged, the gap L depends on the length of the photo filmstrip 102. FIG. 11 is a graph of a condition of breakage of the photo filmstrip 102 upon reverse winding of the photo filmstrip 102 having the above-described shape. In the graph, a horizontal axis is determined to take the available frame number of the photo filmstrip 102, or the maximum number of frames photographable to the photo filmstrip 102. A right-hand vertical axis is determined to take bending force BF (in grams) applied to the innermost turn of the photo filmstrip 102 in the opposite rotational direction. A left-hand vertical axis is determined mined to take the gap L (in mm). The photo filmstrip 102 has the interval t2=0.5 mm, and thickness of 142 .mu.m. Specific features of the photo filmstrip 102 are as follows:
Tear strength: 30 g/cm.sup.2 in the longitudinal direction; and 45 g/cm.sup.2 in the width direction; PA1 Modulus of elasticity: 590 kgf/mm.sup.2 in the longitudinal direction; and 420 kgf/mm.sup.2 in the width direction; PA1 Elongation: 36% in the longitudinal direction; and 32% in the width direction.
A linear velocity of rotation for reverse winding of the photo filmstrip 102 is 500 mm/sec.
It was observed in FIG. 11 that the gap L decreased according to an increase of the available frame number, and that the bending force BF increases according to the increase of the available frame number. If the photo filmstrip 102 has the gap L being small due to smallness of the available frame number, it is difficult for the photo filmstrip 102 to bend back upon reverse rotation of the spool core 5, so that the bending force BF applied to the photo filmstrip 102 is great. The greatness of the bending force BF is remarkable when the photo filmstrip 102 has the available frame number as great as 31-33 frames. Approximately 20% or more of the photo filmstrip 102 of this length are broken and become unusable. The bending force BF is much greater if the photo filmstrip 102 has the available frame number as great as 34-36 frames. Approximately 70% or more of the photo filmstrip 102 of this length are broken.
Likeliness of breakage of the photo filmstrip 102 upon reverse bend of the photo filmstrip 102 increases according to the smallness of the gap L and thus a great length of the photo filmstrip 102. This likeliness is more conspicuous under low temperature which heightens rigidity of a support material of the photo filmstrip 102, or if the support material has characteristically high rigidity irrespective of the environment. The photo filmstrip 102 is disconnected from the spool core 5 to make it impossible to use the photo film cassette any longer, because the photo filmstrip 102 cannot be developed in the photo film processor even after incidental success of taking exposures to the photo filmstrip 102 in the camera.